Trial of Efficacy and Safety of MC0518 Versus Best Available Therapy in Participants With Steroid-Refractory Acute Graft Versus Host Disease

Inclusion Criteria:

1. Participant had a previous allogeneic HSCT as indicated for non-malignant (includinginborn errors of metabolism, primary immunodeficiencies, haemoglobinopathies, andbone marrow failure syndromes) or hematological malignant disease or neuroblastoma.

2. Participant has been clinically diagnosed with Grade II to IV aGvHD according toHarris et al. A biopsy of the involved organs with aGvHD is encouraged but notrequired.

3. Participant has experienced failure of previous first-line aGvHD treatment (that is,SR-aGvHD), defined as:

• aGvHD progression within 3 to 5 days of therapy onset with >=2 milligram perkilogram per day (mg/kg/day) of prednisone equivalent or

• failure to improve within 5 to 7 days of treatment initiation with >=2mg/kg/day of prednisone equivalent or

• incomplete response after greater than (>) 28 days of immunosuppressivetreatment including at least 5 days with >=2 mg/kg/day of prednisoneequivalent.

4. Male or female participant who is >=28 days and <18 years of age and has a minimumbody weight of 3.2 kilograms (kg) at the Screening Visit.

5. Participant has an estimated life expectancy of >28 days.

6. Participant, if female and of childbearing potential, agrees to use a highlyeffective contraceptive measure starting at the Screening Visit and continuingthroughout the entire trial period.

7. Participant, if a fertile male, agrees to sexual abstinence or to use a condomduring sexual activity with their female partner of childbearing potential orpregnant partner. Additionally, if their partner is a woman of childbearingpotential (WOCBP), then their partner must use an additional highly effectivecontraceptive method during sexual activity starting at the Screening Visit andcontinuing throughout the entire trial period.

8. A written informed consent of the participant's parent(s) / legal guardian(s) (andparticipant's assent, when applicable) has been obtained according to nationalregulations.

Exclusion Criteria:

1. Participant has overt relapse or progression or persistence of the underlyingdisease.

2. Participant has received the last HSCT for a solid tumor disease other thanneuroblastoma.

3. Participant has graft-versus-host disease overlap syndrome.

4. Participant has received systemic first-line treatment for aGvHD other than steroidsand a prophylaxis with other than calcineurin inhibitors, mammalian target ofrapamycin (mTOR) inhibitors, anti-thymocyte globulin, mycophenolate mofetil,methotrexate, abatacept, or cyclophosphamide. Note: In vitro or in vivo graftmanipulation to prevent graft-versus-host disease (example, T-cell depletion) duringHSCT is permitted. Restart of initial prophylaxis with calcineurin inhibitors,mammalian target of rapamycin inhibitors, or mycophenolate mofetil after aGvHD onsetis permitted.

5. Participant has received prior mesenchymal stromal cell (MSC) treatment, includingMC0518/Obnitix®.

6. Participant has a known pregnancy (as confirmed by a positive pregnancy test resultat the Screening Visit) and / or is breastfeeding.

7. Participant has a known hypersensitivity to MC0518 and / or its excipients (dimethylsulfoxide, human serum albumin, isotonic sodium chloride solution).

8. Participant has a known hypersensitivity or any contraindication to theInvestigator's choice BAT (extracorporeal photopheresis, anti thymocyte globulin,etanercept, infliximab, or ruxolitinib) and / or its excipients. For a list ofexcipients please refer to the respective Summary of Product Characteristics.

9. Participant has an underlying or current medical or psychiatric condition that, inthe opinion of the Investigator, would interfere with the evaluation of theparticipant.

10. Participant has an uncontrolled infection (examples, sepsis or multi-organ failure)including significant bacterial, fungal, viral, or parasitic infection requiringtreatment.

11. Participant has received treatment with any other investigational agent within 30days or 5 half-lives (whichever is longer) before the Screening Visit.